The aryl hydrocarbon (Ah) receptor (AhR) is a ligand inducible transcription factor, a member of a so-called basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) superfamily. Upon binding to its ligand, AhR mediates or interacts with a series of biological processes as well as some adverse effects including cell division, apoptosis (programmed cell death), cell differentiation, actions of estrogen and androgen, adipose differentiation, hypothalamus actions, angiogenesis, immune system stimulation or suppression, teratogenicity, tumorigenicity, tumor initiation, tumor promotion, tumor progression, chloracne, wasting syndrome, and actions of other hormonal systems beside the expression of genes of P450 family and others[1,2,3,4,5,6,7,8]. The liganded receptor participates in biological processes through translocation from cytoplasm into nucleus, heterodimerization with another factor named Ah receptor nuclear translocator, attachment of the heterodimer to the regulatory region termed Ah response element of genes under AhR regulation, and then either enhancement or inhibition of transcription of those genes.
The AhR happens to be able to bind, with different affinities, to several groups of exogenous chemicals (thus artificial ligands) such as polycyclic aromatic hydrocarbons exemplified by 3-methylchoranthrene (3-MC) and halogenated aromatic hydrocarbons typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The receptor system has been studied so far with its artificial ligands. While studies with those AhR artificial ligands helped in advancing our understanding toward the receptor system, thorough elucidation of the physiological roles the system plays and the potential therapeutic benefits the system may offer are impossible without identification of the AhR physiological ligand. As the first step toward this goal, an endogenous ligand for the receptor has been identified. The endogenous ligand, or physiological ligand, or natural hormone, for the AhR was identified as 2-(1′H-indole-3′-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE)[9,10].
Even though most of the artificial ligands for AhR are environmental toxins[1,2,3] and thus cannot be used as therapeutic agents, for the purpose of understanding functions of liganded AhR, its artificial ligands such as TCDD, 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF), 8-methyl-1,3,6-trichlorodibenzofuran (8-MCDF), and those derived from indole or tryptophan were used to reveal that the liganded AhR was able to inhibit the metastasis of prostate tumors in a strain of transgenic mice[11] and the growth of carcinogen induced rat mammary tumors[12,13,14], human breast tumor cell xenografts[15,16], and tumors caused by gene mutations [17].
As a natural ligand for AhR, ITE is an excellent agent in targeting precisely and specifically the receptor. The consequence of the targeting, however, is unpredictable from what we have learned so far from the behaviors of those artificial ligands for AhR, with some results showing anitcancer potentials[12,13,14,15,16] while others tumor initiation, promotion, and progression[8,18,19,20,21]. From the fact that it is antiangiogenic, ITE might be useful in cancer therapy[7]. The property of antiangiogenesis alone, however, will not automatically qualify ITE as an effective anticancer agent. There are countless examples proving the point[22,23,24,25]. Many antiangiogenic agents failed to perform as therapeutic agents[22] and lots of others even accelerated tumor invasion and metastasis[26,27] due probably to the stress produced by the agents: limited supply of oxygen and nutrients to the tumors. The antiangiogenic therapy is actually a concern since it may even reduce overall survival, the golden standard for cancer therapy, due to possibly an accelerated metastasis[23]. From the anticancer property of liganded AhR revealed by its artificial ligands[12,13,14,15,16], there is no guarantee that ITE, once bound to the same receptor, may also be able to do even partially what these artificial ligands could do without mentioning the fact that most of those artificial ligands or their metabolites may be highly toxic to those cancer cells being tested. In that sense, those artificial ligands or their metabolites may merely serve as non-discriminative cytotoxic agents, killing cancer cells, not even the results of targeting the Ah receptor.
In addition, a critical factor determines what a liganded receptor will do is the final three dimensional (3D) structure the liganded receptor assumes since it is the 3D structure that dictates how many different cellular factors the liganded receptor will interact with and how these interactions should be carried out to conduct the processes of life. The final 3D structure the liganded receptor assumes is, in turn, solely shaped by the 3D structure of the ligand for the receptor in a given biological system. That is the fundamental basis explaining why the 3D structure of a ligand is so crucial in directing its receptor mediated biological and pharmacological processes. Furthermore, ligands with different structures metabolize differently and their different metabolites will certainly interfere with the biological processes differentially. It is obvious, therefore, that ligands with different 3D structures could then certainly lead to completely different biological consequences even if they can bind to the same receptor.
The validity of the point can be easily established through theoretic reasoning above and of course through illustration of literature data also. For example, even though both TCDD and 6-formylindolo[3,2-b]carbazole (FICZ) are high affinity ligands for AhR, TCDD is found to stimulate Treg cell differentiation, thus suppressing the immune system, while FICZ promote Th17 cell differentiation, thus stimulating the immune system[28]. In another example, both TCDD and ITE are high affinity ligands for AhR but while TCDD induced cleft palate, hydronephrosis, and thymic atrophy, ITE did not do any of these[29]. More examples can be easily found in the literature[30,31,32,33,34]. It is, therefore, not obvious at all that ITE will be a good anticancer agent from those studies with artificial ligands for AhR to show their anticancer property[12,13,14,15,16] or even the study with ITE to prove its antiangiogenic property[7] without an extensive research program with different experimental models and systems to find out a clear answer.
The situation prompted us to investigate whether ITE or one of its structural analogs could be used efficaciously and safely in treating or eradicating cancer. The present invention fully discloses a method of using the newly discovered endogenous Ah receptor ligand ITE or one of its structural analogs as a therapeutic agent in cancer intervention or eradication.